In the semiconductor industry, in many applications silicon wafers and the like are held flat through advantageous use of vacuum chucks. As known in the art, such chucks are typically comprised of an aluminum block or the like into which a plurality of concentric 1-2 mm grooves have been machined. Each of the grooves is connected to a controllable source of vacuum pressure. Placing a silicon wafer on the chuck and applying a vacuum to the grooves introduces a suction force against the underside of the wafer thereby gripping it securely to the chuck. Alternatively, chucks manufactured from porous materials have been disclosed, however these currently are only suitable for very thin wafers which are not laminated.
One drawback of systems using such chucks is that the wafers are typically slightly warped during production and therefore do not make a good seal with the surface of the chuck. In many cases this prohibits the development of sufficient suction between the underside of the wafer in order to secure it to the chuck.
What is needed therefore, and an object of the present specification, is an apparatus for creating a seal such that sufficient suction can be developed to secure the wafer to the vacuum chuck.
One application which advantageously uses such vacuum chucks are 3D inspection devices which analyze the surface of the wafer for quality control purposes. In particular, wafers comprising semiconductors which are interconnected with Ball Grid Arrays (BGAs), 3D scanning is used to control the quality of the surface of the wafer and the solder balls or bumps or the like to ensure that their shape and placement falls within certain predefined specification ranges.
In this regard, another drawback therefore is that for very thin wafers, or wafers manufactured from easily pliable materials, the outer groove(s) of the concentric grooves of the vacuum chuck introduces a bending force on the wafer which is not held in chuck by a subsequent groove. As a result the outer edge of the wafer flexes upwards or downwards. In some applications, in particular those taking advantage of 3D scanning, this bending is of such significance that the 3D scanning is no longer able to be performed. In other cases, the bending force is so great that a suction force cannot be brought to bear on the wafer.
What is needed therefore, and also an object of the present specification, is a vacuum chuck which minimises the upward or downward bending of the outer edge of the wafer.